1. Field of the Invention
This invention relates to a hydraulic control system for an automatic transmission used in an automobile, and more particularly to a throttle pressure generator of a hydraulic control system for an automobile transmission.
2. Description of the Prior Art
To avoid power loss of an oil pump, a line pressure generating valve controls the line pressure sent to each frictionally engaging device in relation to the throttle pressure corresponding to a throttle position in an intake path. Also, a shift valve controls a shift point in relation to a governor pressure corresponding to the throttle pressure and vehicle speed to provide a proper value of drive torque. While the shift point is altered by changing a throttle cam to have a common hydraulic control system adapted to various types of engines and vehicles, conventional hydraulic control systems are provided with only one throttle valve and a common throttle pressure is sent to the line pressure generating valve and the shift valve. When the throttle cam is selected to provide the proper shift point, line pressure characteristics become undesirable so that a shock in speed change is disadvantageously increased.
While the publication of Japanese Utility Model Laid-open Sho No. 58-135547 discloses a hydraulic control system comprising a throttle valve for producing a throttle pressure used for a control pressure of the shift valve and another throttle valve for producing another throttle pressure used for the back pressure of a piston of an accumulator, the hydraulic control system does not have any controlling valves for controlling a line pressure in relation to an intake throttle position, and any concept of controlling the shift valve and the line pressure controlling valve by another throttle pressure is not shown in the prior art.